1. Field of the Invention
The invention relates to a method for installing and operating a mass flowmeter operating according to the Coriolis principle having a measuring tube, wherein the measuring tube is excited to vibrate, a phase shift of the vibration of the measuring tube which is dependent on the mass flow of the measuring tube or, respectively, a time difference (td) corresponding to this is determined, the temperature of the measuring tube is determined and the corresponding mass flow is calculated using the determined time difference (td) and the determined temperature (T) by means of a computation rule. Furthermore, the invention relates to a mass flowmeter operating according to the Coriolis principle that is installed in such a manner that the above method can be carried out.
2. Description of Related Art
Mass flow meters operating according to the Coriolis principle have been known for a long time and have been used in widely differing ranges of technology, especially in industrial process engineering. Mass flow meters operating according to the Coriolis principle are constructively differently configured; they can consist of one single or a plurality of straight or curved tube or tubes, which, however, is not important in connection with the present invention. When a mass flow meter comprising “a” measuring tube is mentioned hereinafter, this should not be understood as restrictive but rather, the associated teaching can easily be applied to mass flow meters having a plurality of measuring tubes.
Regardless of their specific embodiment, mass flow meters operating according to the Coriolis principle have in common that their measuring tube is excited to vibrate by a—generally centrally arranged—vibration generator. In the state of the measuring tube with no through-flow, the measuring tube vibrates symmetrically about the excitation point. Depending on the flow of a medium through the measuring tube—and therefore depending on the mass flow of the medium through the measuring tube—the form of the vibration changes on both sides of the excitation point and therefore becomes asymmetrical if symmetry had existed previously. The vibration components detected by the measured value sensor on both sides of the excitation point are phase-shifted, the phase shift being proportional to the actual mass flow. The phase shift of the vibrations detected on both sides of the excitation point naturally corresponds to a time difference, i.e. for example the time difference between the zero crossing of the measuring tube on one side of the excitation point and the zero crossing of the measuring tube on the other side of the excitation point of the measuring tube.
Apart from the general desire to improve a measuring device with regard to its accuracy, particularly stringent accuracy requirements are imposed on mass flowmeters in specific cases of application e.g. in applications requiring calibration which require Coriolis mass flow meters that can be calibrated; this is the case, for example, in the monitored distribution of fluid media—custody transfer. In this case, the required accuracies can be in the per-mill range.
Mass flow meters, including ones that can not be calibrated, are usually calibrated at the factory, i.e. in a test rig exposed to a defined mass throughput (standing/flying start-and-stop method), wherein a calibration factor is calculated from the mass flow determined by the mass flowmeter and the actual mass flow pre-defined with a high accuracy, this calibration factor is taken into account within a computation rule, wherein the computation rule converts the time difference present as a measured quantity into a corresponding value for the mass flow with the aid of the calibration factor. Such a computation rule looks like the following:{dot over (m)}=KRtd.  (1)
The calibration of the mass flowmeters is, in this case, made at a fixed, well-defined temperature, namely the reference temperature, which is close to the expected operation temperature, which can, for example, be 20° C.
Experience shows that the accuracy of the measurement result at an operating temperature of the Coriolis mass flowmeter differing from the reference temperature can be poorer, possibly even departing from the still-accepted accuracy range. In order to maintain the measurement accuracy despite an operating temperature differing from the reference temperature, it is known from the prior art to determine the temperature T of the measuring tube and quite especially take into account the dependence of Young's modulus E of the material of the measuring tube on the temperature of the measuring tube within the scope of the computation rule. This is based on the fact that the vibration properties of the Coriolis measuring tube depend appreciably on the Young's modulus E of the material of the measuring tube and thus a temperature dependence of the Young's modulus immediately has the effect as a temperature dependence of the vibration property of the measuring tube. This relation is known and, for example, can be seen using the formulation according to Equation 2:
                              m          .                =                                            C              ·                              EI                p                                                                    ψ                (                                                      x                    1                                    1                                )                            ⁢                              1                3                                              ⁢                                    t              d                        .                                              (        2        )            
In Equation 2,                C is a constant        E is Young's modulus of the material of the measuring tube        Ip is the moment of inertia of the measuring tube,        
            I      p        =                  π        64            ⁢              (                              D            4                    -                      d            4                          )              ,where D is the outside diameter and d is the inside diameter of the measuring tube,                ψ(·) is a function along the distance of the measuring tube, where the value at the location of the sensor        
      x    l    lis of interest.
However, it has been found that in the case of large temperature fluctuations, compensation or allowance for the temperature dependence of Young's modulus of the measuring tube does not yield a sufficient result with regard to measuring accuracy. This particularly affects applications of mass flowmeters in which extremely cold media (e.g. liquid nitrogen, boiling point: −195.80° C.;) or extremely hot media flow through the measuring tube.